The present invention relates to a hydraulic system for a variable assist power steering system.
A power steering system can be characterized as operating under three driving conditions. Firstly during straight ahead driving at medium to high speed, the power requirements on the steering system are extremely low and the degree of power assistance provided by the steering gear should correspondingly minimized to permit the feedback of road "feel" from the tires to the driver. Secondary during medium and high speed passing and cornering maneouvres, a progressive increase in the level of power assistance with driver input torque is desirable. Nevertheless moderate driver input torques should still be maintained in order that the driver can feel adequately the dynamic state of the vehicle. Thirdly, and lastly, during low speed or parking maneuvers, the power requirements on the steering system may be large and fidelity of the steering system in terms of transmitting road feel is of little importance. Under these circumstances it is generally desirable to offer large degrees of power assistance, thereby minimizing the input torque required to be furnished by the driver.
The demands for optimum valve characteristics during the above three driving conditions conflict. There are known three representative systems which have attempted to avoid the conflicting demands of the first and third driving conditions, namely the need to achieve a low level assistance for high to medium speed on-center driving while having high levels of assistance for low speed and parking maneouvres.
Firstly, there is known an oil flow controlling system which exploits the fact that the degree of power assistance varies with the flow of oil passing through valve. For example, in one such widely used system, the power steering pump is caused to reduce the flow of oil as vehicle speed increases. However, this adversely affects valve performance in the second driving condition above, namely medium to high speed passing and cornering maneuvers, where progressive valve response is impaired due to the low oil flow. Also, in the event that such a steering maneuver requires rapid turning of the steering wheel, the lower pump flow may be inadequate, rendering the power assistance momentarily inoperative. If a pump is used which can discharge oil flow high enough to offer a sufficiently high power assistance during rapid turning at high vehicle speed, the overabundance of oil flow is discharged during low speed or parking maneuver. Such a pump with a large discharge capacity, however, causes an increase in the amount of heat emitted from the whole hydraulic system, thereby requiring an arrangement to dissipate heat. This results in an increase in manufacturing cost.
Secondary, a reaction controlling system is also known wherein a reaction hydraulic pressure which gives a presetting force is controlled. This system requires additional component parts for a reaction chamber and a reaction piston, and a hydraulic pressure selector valve, resulting in a bulky construction and a complicated piping. Thus, a large installation space is needed, causing an increase in manufacturing cost.
Lastly, the most satisfactory method of matching valve performance in all three of the above mentioned conditions is modulating the valve characteristic with vehicle speed. A system which provides for better modulation of power assistance with vehicle speed is disclosed in U.S. Pat. No. 4,561,521 and can be seen to employ a rotary valve with primary and secondary valve portions. A speed sensitive valve is used to control oil flow from the pump to the secondary valve portion so that at high vehicle speeds a parallel flow path is provided between the rotary valve and the pump as oil is distributed to both primary and secondary valve portions. At low vehicle speeds, the speed sensitive valve restricts the flow of oil from the pump to the secondary valve portion. During parking maneuvers, the primary valve portion acts alone in the normal manner and the secondary valve portion is vented and not fed with oil from the pump. A change from a high level of power assistance to a low level of power assistance, and conversely, is effected by a variable force solenoid which is used to establish a parallel flow path from the pump to the secondary valve portion through a variable flow orifice. A speed sensing module controls the solenoid to open and close a variable orifice valve thus providing gradual changes in the level of power assistance as the vehicle speed changes. The rotary valve used in this power steering system includes a valve housing having a circular opening which receives a valve sleeve. Positioned within the valve sleeve is an inner valve. The inner valve is formed with a primary set of longitudinal grooves forming a primary valve section, and also with a secondary set of longitudinal grooves forming a secondary valve section. The primary and secondary sets of longitudinal grooves register with primary and secondary sets of internal grooves formed in the internal wall of the valve sleeve, respectively. The primary and secondary sets of internal grooves are difficult to machine and require skilled labour because they have to be formed in the cylindrical internal wall of the valve sleeve with high precision. This results in increased production steps and manufacturing cost.
A main object of the present invention is to provide a variable assist power steering system of the less expensive and less complicated type wherein a pump with a large discharge capacity is not required and a secondary valve portion is not required either to achieve a low level power assistance for high to medium speed on-center driving while having high levels of power assistance for low speed and parking maneuvers and an adequate level of power assistance for medium to high speed passing and cornering maneuvers.